Fiber optic transmission uses the same basic elements as copper-based transmission systems. Specifically, both systems utilize transmitters and receivers and a medium by which a signal is passed from the transmitter to the receiver. Instead of a copper wire, optic transmission systems use an optical fiber.
Typically, a fiber optic transmitter uses a laser diode or other light emitting device (LED) to optically encode information and generate an optical output at various light wavelengths, e.g., 850 nm, 1310 nm, 1550 nm etc. The optical fiber connects the transmitter to a receiver which then converts the optical signal to an electrical signal. The optical fiber may be either single-mode or multi-mode. Typical receivers incorporate optoelectronic transducers such as photodiodes to convert the optical signal to an electrical signal. A data recovery circuit then converts the data back into its original electrical form.
In order to increase transmission rates, wavelength division multiplexing (WDM) was developed for sending several different signals through a single fiber at different wavelengths. WDM components allow these separate signals to be joined into a composite output signal for transmission and then separated back into their original signals at the receiver end. Coarse wavelength division multiplexing (CWDM) is typically used up to 16 channels and dense wavelength division multiplexing (DWDM) allow up to several hundreds of signals to be combined into a single fiber. DWDM allow a multiple wavelength transmission in the C-Band (1550 nm) and more recently in the S-Band and L-Band as well. CWDM schemes have been used in many wavelength bands including near 850 nm, 1300 nm and all bands at 1500 nm.
However, despite the ability to transmit multiple signals over a single optical fiber, the transmitters of each signal continue to be single wavelength devices. Specifically, a typical transmitter includes a laser diode or other light emitting device coupled to a modulator which then outputs the modulated signal either directly or indirectly to the optical fiber. Typically, an isolator and various lens arrangements are disposed between the laser diode and the modulator. Also, a tap photodiode may also be included to measure the outputted signal.
The use of separate transmitters for each signal increases the size and complexity of the wavelength division multiplexing apparatus. Not only does the use of separate components for each signal add to the manufacturing time and cost, the incorporation of additional components into any optical transmission system further increases the possibility of defective products because of the precise alignment that is required between each component. Thus, reducing the number of components not only reduces the size and cost of each assembly, reducing the number of components also increases manufacturing efficiency.
As a result, there is a need for improved optical transmitter designs which are capable of transmitting multiple signals at multiple wavelengths and which reduce the total number of components needed to perform such a wavelength division multiplexing transmission.